Considerable effort has been devoted for developing rapidly and widely tunable wavelength laser sources for optical reflectometry, biomedical imaging, sensor interrogation, and tests and measurements. A narrow line width, wide-range and rapid tuning have been obtained by the use of an intra-cavity narrow band wavelength scanning filter. Mode-hopping-free, single-frequency operation has been demonstrated in an extended-cavity semiconductor laser by using a diffraction grating filter design. Obtaining single-frequency laser operation and ensuring mode-hop-free tuning, however, may use a complicated mechanical apparatus and limit the maximum tuning speed. One of the fastest tuning speeds demonstrated so far has been limited less than 100 nm/s. In certain applications such as biomedical imaging, multiple-longitudinal mode operation, corresponding to an instantaneous line width as large or great than 10 GHz, may be sufficient. Such width may provide a ranging depth of a few millimeters in tissues in optical coherence tomography and a micrometer-level transverse resolution in spectrally-encoded confocal microscopy.
A line width on the order of 10 GHz can be achievable with the use of an intra-cavity tuning element (such as an acousto-optic filter, Fabry-Perot filter, and galvanometer-driven diffraction grating filter). However, the sweep frequency previously demonstrated has been less than 1 kHz limited by finite tuning speeds of the filters. Higher-speed tuning with a repetition rate greater than 25 kHz may be needed for video-rate (>30 frames/s), high-resolution optical imaging in biomedical applications.
A wavelength-swept laser that uses a diffraction grating and polygon scanner has provided high-speed wavelength tuning up to 20,000 nm/ms. While the high-speed polygon based wavelength-swept light source enabled high-speed imaging as fast as 200 frames/s, wavelength tuning rate as fast as 20,000 nm/ms keeping an instantaneous line width narrower than 0.15 nm has already reached to the limit of the polygon based wavelength-swept laser. In addition, a tuning rate of 7000-nm/ms is achieved with 65 mW of power over a wavelength range of 120-nm and with an instantaneous line-width ˜0.07 nm.
Accordingly, for faster tuning and especially for wide wavelength tuning range (˜200 nm) and (or) narrow instantaneous line width (˜0.07 nm) at fast tuning rate, it may be beneficial to provide a further wavelength scanning filter and laser scheme/procedure and/or overcome at least some of the deficiencies described herein above.